Advantages: + Waitfree and fast producers. One XCHG is maximum what one can get with multi-producer non-distributed queue. + Extremely fast consumer. On fast-path it's atomic-free, XCHG executed per node batch, in order to grab 'last item'. + No need for node order reversion. So pop operation is always O(1). + ABA-free. + No need for PDR. That is, one can use this algorithm out-of-the-box. No need for thread registration/deregistration, periodic activity, deferred garbage etc. Disadvantages: - Push function is blocking wrt consumer. I.e. if producer blocked in (*), then consumer is blocked too. Fortunately 'window of inconsistency' is extremely small - producer must be blocked exactly in (*). Actually it's disadvantage only as compared with totally lockfree algorithm. It's still much better lock-based algorithm. - The algorithm is not linearizable. struct mpscq_node_t
};
struct mpscq_t
{ mpscq_node_t* volatile head; mpscq_node_t* tail; };
void mpscq_create(mpscq_t* self, mpscq_node_t* stub)
{ stub->next = 0; self->head = stub; self->tail = stub; }
void mpscq_push(mpscq_t* self, mpscq_node_t* n)
{ n->next = 0; mpscq_node_t* prev = XCHG(&self->head, n); // serialization-point wrt producers, acquire-release prev->next = n; // serialization-point wrt consumer, release }
mpscq_node_t* mpscq_pop(mpscq_t* self)
{ mpscq_node_t* tail = self->tail; mpscq_node_t* next = tail->next; // serialization-point wrt producers, acquire if (next) { self->tail = next; tail->state = next->state; return tail; } return 0; } |
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